1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an array substrate having a color filter-on-thin film transistor (COT) structure.
2. Discussion of the Related Art
In general, since flat panel display devices are thin, light weight, and have low power consumption, they are used in portable display devices. Among the various types of flat panel display devices, liquid crystal display (LCD) devices are commonly used in laptop computers and desktop computer monitors because of their superior resolution, color image display, and display quality.
In the LCD devices, optical anisotropy and polarization characteristics of liquid crystal molecules are utilized to generate images, wherein liquid crystal molecules have specific alignment directions that result from their inherent properties. The specific alignment directions of the liquid crystal molecules may be modified by application of an electric field. Thus, due to the optical anisotropy, incident light is refracted according to the alignment of the liquid crystal molecules.
The LCD devices include upper and lower substrates each having electrodes that are spaced apart and face each other, and a liquid crystal material interposed between the upper and lower substrates. Accordingly, when a voltage is applied to each of the electrodes of the upper and lower substrates, the alignment direction of the liquid crystal molecules is changed in accordance with the applied voltage, thereby displaying images. By controlling the applied voltage, the LCD device provides various light transmittances to display image data.
The liquid crystal display (LCD) devices are commonly employed in office automation (OA) and video equipment due to their light weight, slim design, and low power consumption. Among the different types of LCD devices, active matrix LCDs (AM-LCDs), which have thin film transistors and pixel electrodes arranged in a matrix form, provide high resolution and superiority in displaying moving images.
In general, the LCD device displays an image using light emitted from a backlight device that is positioned under the LCD panel. However, the LCD only utilizes 3˜8% of the incident light generated from the backlight device, thereby providing inefficient optical modulation. Thus, the LCD device using the backlight device usually consumes a significant amount of electrical energy in order to provide light of reasonable brightness.
In order to overcome the high power consumption, there is a need for a transflective LCD device that utilizes ambient light and artificial light generated from the backlight device. Specifically, the transflective LCD device may be used during daytime hours as well as nighttime because the transflective LCD device can be changed to operate in one of a transmissive mode and a reflective mode depending on the desired condition of operation. The transflective LCD device includes a reflector within each pixel where a transparent electrode mainly exists to electrically communicate with the transparent electrode, and then function as a pixel electrode with the transparent electrode.
FIG. 1 is a cross sectional view of a transflective liquid crystal display device according to the related art. In FIG. 1, first and second substrates 10 and 50 are spaced apart and face each other, wherein a front surface of the first substrate 10 includes a thin film transistor T having a gate electrode 12, a semiconductor layer 16, and source and drain electrodes 18 and 20. In addition, a gate insulation layer 14 is formed on the first substrate 10 and is interposed between the gate electrode 12 and the semiconductor layer 16 in the thin film transistor T, and a first passivation layer 22 is formed on the gate insulation layer 14 to cover the thin film transistor T. A reflector 24 that reflects incident light is formed on the passivation layer 22 within an area where a reflective portion is defined within a pixel region P. A second passivation layer 26 is formed over the first substrate 10 to cover the reflector 24, and the first and second passivation layers 22 and 26 and the reflector 24 together include a drain contact hole 28 that exposes a portion of the drain electrode 20. A pixel electrode 30 is disposed on the second passivation layer 26 within the pixel region P, which is formed of a transparent conductive material, and contacts the drain electrode 20 through the drain contact hole 28. In addition, a data line 21 is formed on the gate insulation layer 14 and is connected with the source electrode 18. Although not shown in FIG. 1, a gate line is formed with the gate electrode 12 on the first substrate 10 and defines the pixel region P while perpendicularly crossing the data line 21.
In FIG. 1, a black matrix 52 is formed on a rear surface of the second substrate 50 to correspond in position to the data line 21 and the gate line (not shown). A color filter layer 54 having red, green, and blue colors is formed on the rear surface of the second substrate 50 while covering the black matrix 52. In addition, a common electrode 56 is disposed on a rear surface of the color filter layer 54, and is formed of the same material as the pixel electrode 30, such as a transparent conductive material. Moreover, a liquid crystal layer 70 is interposed between the pixel electrode 30 and the common electrode 56. Accordingly, the pixel region P is divided into a reflective portion, which corresponds to the reflector 24, and a transmissive portion, which corresponds to the pixel electrode except for the reflective portion.
The reflective LCD device, as shown in FIG. 1, is fabricated by sequential processes including: a gate process (forming the gate electrode and line); an active process (forming the semiconductor layer); a source/drain process (forming the source and drain electrodes and the data line); a first contact hole process (forming the first passivation layer); a reflective process (forming the reflector); a second contact hole process (forming the second passivation layer); a transmissive process (forming the pixel electrode); a black matrix process (forming the black matrix); a color filter process (forming the red, green, and blue color filters); a common electrode process (forming the common electrode); and an aligning process (attaching the first and second substrates and interposing the liquid crystal therebetween). Accordingly, the fabrication processes for manufacturing the reflective LCD device of FIG.1 is extremely complicated. In addition, a second contact hole process for forming the second passivation layer 26 may be necessary since the second passivation layer 26 prevents electrochemical reaction between the reflector 24 and the pixel electrode 30.
The reflector 24 commonly includes aluminum (Al) or silver (Ag) material having superior reflectivity, and the pixel electrode 30 is commonly formed of the transparent conductive material, such as indium-tin-oxide (ITO). When the reflector 24 and the pixel electrode 30 are dipped together into a solution, the indium ion (In3+) of ITO obtains electrons and becomes an indium (In) metal and aluminum (Al) of the reflector loses electrons and becomes the aluminum ion (Al3+). As a result, the transparent conductive material (ITO) is damaged, thereby losing its transparency and becomes darkened. Therefore, the second passivation layer 26 is necessary between the reflector 24 and the pixel electrode 30 in order to isolate the reflector 24 from the pixel electrode 30.
The transflective LCD device of FIG. 1 may be operated in both the reflective mode and the transmissive mode. In the reflective mode, incident light L1 is reflected from the reflector 24 and is directed toward the second substrate 50, whereby the incident light encounters and passes through the color filter layer 54 twice. Conversely, artificial light L2 generated from the backlight device (not shown) only passes through the color filter layer 54 once. Accordingly, the light L1 and L2 colored by the color filter layer 54 to display color images have different light paths in the reflective mode and the transmissive mode, respectively. Because of these different light paths, color reproduction in the reflective portion is different from that in the transmissive portion even though the same color filter is utilized.
To overcome the problem of different color reproduction, the color filter in the reflective portion is formed to have a one-half thickness than that in the transmissive portion. Thus, the light path of incident light is shortened when the incident light passes through the color filter in the reflective mode.
FIG. 2 is a partial cross sectional view of a transflective LCD device according to the related art. In FIG. 2, a transparent interlayer 82 is formed on a rear surface of a upper substrate 80, and a color filter 84 is formed on the rear surface of the upper substrate 80 to cover the transparent interlayer 82. The transparent interlayer 82 corresponds to a reflective portion where a reflector 94 is disposed. A first portion 84a of the color filter 84 has a first thickness dl that corresponds to the reflective portion, and a second portion 84b of the color filter 84 has a second thickness d2 that corresponds to the transmissive portion. Since the transparent interlayer 82 is formed within the reflective portion between the substrate 80 and the color filter 84, the first thickness d1 is less than the second thickness d2 by as much as the thickness of the transparent interlayer 82. When incident light LL1 is reflected on the reflector 94, the incident light LL1 transits twice through the first portion 84a of the color filter 84 whose thickness is almost one-half that of the second portion of the color filter 84. Therefore, the light path of incident light LL1 is the same as that of artificial light LL2 that passes through the second portion 84b. 
However, since the transparent interlayer 82 is provided to lower the thickness of the first portion 84a of the color filter 84, supplementary processes for forming the transparent interlayer 82 are required. Second, it is essential to form the color filter 84 to be planar in order to form a common electrode on its surface. Accordingly, much more complicated process steps are necessary for the transflective LCD device, and fabrication costs of transflective LCD device will increase.